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Test-Flight Card, Pitot Static and IAS Check

Flight Test Card
Test-flight card format

Flight Test Card
The test aircraft, an RV-9A

Flight Test Card
The test aircraft, an RV-9A

Flight Test Card
The test aircraft, an RV-9A

By Jack Dueck, EAA Homebuilt Aircraft Council, EAA 337912

I now have several hours of flight time logged on my new homebuilt. I am becoming quite comfortable with basic flight maneuvers and also have several hours working the engine to help break in seating of the rings. It's time to proceed with the exploration of the flight envelope.

In Canada, we normally would need to fly off a total of 25 hours of trouble-free flight under a flight authority before receiving our special certificate of airworthiness for our amateur-built aeroplane. During this test period, we want to explore the flight envelope. And so we will look at the various simple flight tests we can do, the data we can collect, and the reduction of this data to useful information contained in our own pilot's operating handbook (POH).

Pitot Static and IAS System Check

Process:

To gather appropriate data, we'll fly a square pattern and average the indicated airspeed readings over all four quadrants. We'll fly the pattern at different airspeeds, and best results will be obtained if we can do these runs at a zero wind and at constant altitude and temperature conditions.

The quadrant should be based on GPS headings of 360, 270, 180, and 90 degrees. We should maintain a constant altitude for all tests and record both the pressure altitude and the temperature in order to reduce our data to standard conditions. (Remember to reset the altimeter before returning to the airfield.)

We select target speeds at the low end of, say, 1.3 stall and throughout the range to the high end of maximum straight and level flight less 10 knots/mph. We will record several test values at different flap extension positions because the pitot-static accuracy will be affected by the airflow around the aircraft.

Maintaining the same indicated airspeed at a constant altitude for 15 seconds or so before taking the indicated airspeed (IAS) reading will result in greater accuracy.

Procedure:

Climb to a suitable altitude. I used 6,000 ASL and set up for five test runs. Allow the engine temperatures to stabilize and record the engine readings. Set up for your first test run, flaps up, about 1.3 x flaps-up stall speed. (I used 70 knots indicated airspeed [KIAS].) Fly a heading of 360 degrees on the GPS at constant airspeed and altitude, allowing the aircraft to stabilize at these conditions. Record the ground speed as shown on the GPS. Now repeat this for the three remaining quadrant GPS headings.

Next, set up and conduct the remaining test runs in the same manner at the same pressure altitude setting on your altimeter. I flew the following additional configurations:

  • Takeoff flaps and 1.3 x flap stall speed (65 KIAS)
  • Full flaps and 1.3 x flap stall speed (65 KIAS)
  • Flaps up and maximum speed less 10 percent (140 KIAS)
  • Flaps up and intermediate speed (110 KIAS)

Be sure to conduct all test runs at the same pressure altitude and record the temperature at every test run. I also recorded engine readings after the high-speed test run.

Data reduction:

To simplify the reduction of the test data recorded into useful form, I used a simple spreadsheet to work out the required data. Using an E6B computer, I converted the indicated airspeed (knots) to true airspeed (knots) at standard conditions. These values are recorded in the row "KTAS". Next I averaged the IAS obtained in the four quadrants and recorded these in the row "Average". The final row, "Error", shows the difference between the average GPS readings and the true airspeed (KTAS).

In all instances the error is less than 7 percent, and I am left with the decision of whether I live with these readings or move my pitot-static ports to improve the accuracy.

KIAS vs. KTAS, CFBJJ, RV-9A

  Flaps up Half flaps Full flaps Flaps up Flaps up
        140 KIAS 110 KIAS
Press Alt 6,000 6,000 6,000 6,000 6,000
OAT 21 20.5 20 18.5 18
KIAS 70 65 65 140 110
KTAS 79 73 73 157 124
           
360 85 79 78 148 120
90 78 73 72 142 115
180 83 77 77 145 118
270 90 83 84 150 124
Average 84 78 77.75 146.25 119.25
           
Error -5 -5 -4.75 100.75 4.75

In evaluating these results, it is interesting that with a higher angle of attack, my airspeed readings are lower than actual, but as the airspeed increases and the angle of attack decreases, the error is reflected in the opposite direction with my indicated airspeed higher than actual. Also, since the points on the graph are far apart, the accuracy of the data is suspect, and an additional flight test should probably be flown with various intermediate airspeeds to give a more defined resultant graph.

How can I use this information?

The graph below shows the readings for the various airspeeds flown. These readings are based on standard conditions, and I can use them to calculate my true airspeed at various conditions. I simply need to add or subtract the error component to my indicated airspeed values to arrive at a calibrated airspeed (CAS). Then using this value, convert it to ambient conditions of pressure and temperature.

From this graph, I develop a simple chart that I will install in my POH that will place the necessary information at my immediate use. (See below.)

KIAS vs. KCAS

KIAS Error KCAS
60 5 65
70 4 74
80 3 83
90 1 91
100 -2 98
110 -5 105
120 -8 112
130 -9 121
140 -11 129

Note: Since the IAS readings are lower than CAS at the lower values, and higher at the higher values, I've changed the sign convention accordingly.

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